1. Field of the Invention
This invention relates to a feedforward-type distortion compensation circuit, and more particularly to a distortion compensation circuit for a linear amplifier used in a radio frequency (RF) band.
2. Description of the Related Art
In a radio frequency band such as a microwave, a feedforward-type distortion compensation circuit is used to remove distortion in an input/output characteristic of an amplifier to realize a high output linear amplifier in order to reduce the power dissipation. The feedforward-type distortion compensation circuit is basically constituted from two signal cancellation circuits, i.e., a distortion detection circuit and a distortion removal circuit as disclosed, for example, in Japanese Patent Laid-Open Application No. Sho-61-121607 (JP, A, 61-121607) by Y. Yajima et al., Japanese Patent Laid-Open Application No. Hei-1-198809 (JP, A, 1-198809) by T. Nojima and Japanese Patent Laid-Open Applications Nos. Hei-4-70203 (JP, A, 4-70203) and Hei-4-83406 (JP, A, 4-83406) by Y. Narahashi et al. The distortion detection circuit includes a main amplifier and detects a distortion component included in the output of the main amplifier from a difference between an input signal to the main amplifier and an output signal of the main amplifier. Meanwhile, the distortion removal circuit includes an auxiliary amplifier and removes distortion by subtracting from the output of the main amplifier the distortion component detected by the distortion detection circuit and amplified by the auxiliary amplifier.
As an example, the feedforward-type distortion compensation circuit disclosed in above JP, A, 4-83406 is described below with reference to FIG. 1.
A distortion detection circuit 1 is constituted from a path (main amplifier path) ML including a main amplifier 8, another path (first delay line path) DL1 including a delay line 46, and a power distributor 5 for distributing an input signal to both of the paths ML and DL1. Meanwhile, a distortion removal circuit 2 is constituted from a path (second delay line path) DL2 including a delay line 47, another path (auxiliary amplifier path) SL including an auxiliary amplifier 20, and a power combiner 17 for combining signals from both of the paths DL2 and SL and outputting a resulted signal. The main amplifier path ML and the second delay line path DL2 are connected to each other by a power distributor 11, and the first delay line path DL1 and the auxiliary amplifier path SL are connected to each other by a power combiner 14. The branch side of the power distributor 11 and the branch side of the power combiner 14 are connected to each other by a signal transmission path BL.
The distortion detection circuit 1 further includes an electrically adjustable variable attenuator 6 and an electrically adjustable variable phase shifter 7. In the example shown in FIG. 1, the variable attenuator 6 and the variable phase shifter 7 are interposed in the main amplifier path ML. Alternatively, the variable attenuator 6 and the variable phase shifter 7 may be interposed in the first delay line path DL1 because the alternative arrangement does not cause a significant difference in characteristic. Similarly, the distortion removal circuit 2 includes an electrically adjustable variable attenuator 18 and an electrically adjustable variable phase shifter 19. In the example shown in FIG. 1, the variable attenuator 18 and the variable phase shifter 19 are interposed in the auxiliary amplifier path SL. Also the variable attenuator 18 and the variable phase shifter 19 may alternatively be interposed in the second delay line path DL2.
Next, operation of the feedforward-type distortion compensation circuit will be described. An input signal applied to an input terminal 26 is inputted to the distortion detection circuit 1 by way of a directional coupler 3 and distributed to the main amplifier path ML and the first delay line path DL1 by the power distributor 5. The signal component which passes through the main amplifier path ML, in which it is amplified by the main amplifier 8, and has distortion is distributed to the second delay line path DL2 and the signal transmission path BL by the power distributor 11. The signal component having passed through the signal transmission path BL and having distortion is combined with the signal component having passed through the first delay line path DL1 by the power combiner 14, and the thus combined signal is outputted to the auxiliary amplifier path SL. In this instance, the variable attenuator 6 and the variable phase shifter 7 are adjusted so that the two signal components to pass through the two paths and be inputted to the power combiner 14, i.e., the first delay line path DL1 and the signal transmission path BL, are equal in amplitude and delay amount but are opposite in phase. As a result, the signal component outputted from the power combiner 14 to the auxiliary amplifier path SL is a difference component between the signal having passed through the delay line 46 and the signal having passed through the main amplifier 8. When the variable attenuator 6 and the variable phase shifter 7 are adjusted in such a manner as described above, the difference component includes only a non-linear distortion component produced by the main amplifier 8, and accordingly, only the non-linear distortion component is outputted from the power combiner 14 to the distortion removal circuit 2.
The signal having passed through the main amplifier path ML and outputted from the power distributor 11 to the second delay line path DL2 of the distortion removal circuit 2 is combined with the signal having passed through the auxiliary amplifier path SL by the power combiner 17. In this instance, the variable attenuator 18 and the variable phase shifter 19 are adjusted so that the transfer characteristics of the two paths from the input terminal of the power distributor 11 to the output terminal of the power combiner 17, i.e., the second delay line path DL2 and the auxiliary amplifier path SL, are equal to each other in terms of amplitude and the delay amount but are opposite in terms of the phase. Since the signal passing the auxiliary amplifier path SL is a distortion component of the output signal of the main amplifier 8 as described above, by adjusting the variable attenuator 18 and the variable phase shifter 19 in this manner, the power combiner 17 combines the output signal of the main amplifier 8 and the distortion component in opposite phases and in equal amplitudes, thereby realizing cancellation of the distortion component in the output of the entire circuit.
The factors which have a significant influence on the distortion compensation performance of the feedforward-type distortion compensation circuit are the adjustment precision of the variable attenuator 6 and the variable phase shifter 7 in the distortion detection circuit 1 and the adjustment precision of the variable attenuator 18 and the variable phase shifter 19 in the distortion removal circuit 2. In short, whether distortion can be removed precisely or not depends upon whether signals can be combined by the distortion detection circuit 1 and the distortion removal circuit 2 satisfying the requirements of equal amplitudes, equal delay amounts and opposite phases. In practice, however, it is not easy to maintain the settings of the variable attenuators 6 and 18 and the variable phase shifters 7 and 19 so as to satisfy the requirements. Even if adjustment is performed by initial control so that the requirements may be satisfied completely, since the characteristic of the main amplifier 8 varies with changes in ambient temperature, small variations in power supply voltage, avariations in input power, aging and so forth, the transfer characteristics of the distortion detection circuit 1 and the distortion removal circuit 2 must follow up the changes in the characteristic of the main amplifier 8.
Therefore, the feedforward-type distortion compensation circuit disclosed in JP, A, 4-83406 mentioned above employs an oscillator 4 for generating a first pilot signal and another oscillator 9 for generating a second pilot signal so that the requirements mentioned above may be satisfied precisely when signals are combined by the distortion detection circuit 1 and the distortion removal circuit 2. In particular, the first pilot signal is coupled to the distortion detection circuit 1 by way of the directional coupler 3 whereas the first pilot signal is extracted by a directional coupler 40 provided in the auxiliary amplifier path SL of the distortion removal circuit 2 and a residual level of the first pilot signal is determined by a selective level meter 43 to detect whether combination of signals satisfying the requirements of equal amplitudes, equal delay amounts and opposite phases is performed by the distortion detection circuit 1. Since the requirements of equal amplitudes, equal delay amounts and opposite phases are satisfied at a higher degree as the residual level detected by the selective level meter 43 becomes lower, the variable attenuator 6 and the variable phase shifter 7 are adjusted by a control circuit 25 so that the residual level may be minimized in order that the requirements described above may be satisfied at the distortion detection circuit 1. Similarly, the second pilot signal is coupled to the main amplifier path ML by way of a directional coupler 10 whereas the second pilot signal is extracted by a directional coupler 41 provided on the output side of the distortion removal circuit 2 and a residual level of the second pilot signal is determined by a selective level meter 44. Then, the variable attenuator 18 and the variable phase shifter 19 are adjusted by the control circuit 25 to minimize the residual level detected by the selective level meter 44. By the adjustment just described, the requirements described above are satisfied also at the distortion removal circuit 2.
The pilot signals for adjusting the distortion detection circuit and the distortion removal circuit are also used in above patent documents JP, A, 61-121607, JP, A, 1-198809 and JP A, 4-70203. Further, T. Nojima et al. discloses in Japanese Patent Laid-Open Application No. Hei-4-344602 (JP, A, 4-364602) a feedforward interference circuit in which a spread spectrum signal is used as a pilot signal.
In the feedforward-type-distortion compensation circuit shown in FIG. 1, in order to remove the first pilot signal component from the output signal thereof, a directional coupler 42 is provided at the output of the distortion removal circuit 2 to extract the first pilot signal, and a residual level of the first pilot signal is detected by a selective level meter 45, and another directional coupler 32 is provided in the auxiliary amplifier path SL. The first pilot signal generated by the oscillator 4 successively passes through electrically adjustable variable attenuator 29 and variable phase shifter 30 and an amplifier 31 and is coupled to the auxiliary amplifier path SL by the directional coupler 32. The variable attenuator 29 and the variable phase shifter 30 are adjusted by the control circuit 25 so that the residual level detected by the selective level meter 45 may be minimized in order to minimize the level of the first pilot signal outputted from an output terminal 27.
As described above, in the feedforward-type distortion compensation circuit disclosed in JP, A, 4-83406, in order to accurately grasp the transfer characteristics of the distortion detection circuit 1 and the distortion removal circuit 2, the first and second pilot signals are coupled to those circuits and the levels of the signals are detected on the output sides of the circuits. However, the detected residual pilot signals do not directly indicate adjustment amounts for variable attenuators and variable phase shifters but are mere parameters indicating incompleteness of the signal combination requirements of equal amplitudes, equal delay amounts and opposite phases. Control amounts for the variable attenuator and the variable phase shifters must be determined based on an estimation algorithm from the residual levels of pilot signals. Consequently, an excessive burden is imposed on the control circuit 25, and moreover a large number of steps are required to reach an optimum adjustment amount. Accordingly, it is difficult for the transfer characteristics of the distortion detection circuit 1 and the distortion removal circuit 2 to follow up a sudden or small variation in characteristic.